The invention relates to thermostable pigments, films and effect coatings, and to mixtures for their production.
Materials having a liquid-crystalline structure with a chiral phase (LC materials) are known. The production of such materials from LC organosiloxanes is described, for example, in U.S. Pat. No. 5,211,877. Pigments containing aligned, three-dimensionally crosslinked substances having a liquid-crystalline structure with a chiral phase (LC pigments), their production and use are also known, for example from DE 42 407 43, which corresponds to U.S. Pat. No. 5,362,315.
One problem in the processing of LC materials, for example in surface finishes, is the varying color of the material depending on the application temperature. In paint systems employing conventional LC materials, for example those produced from LC organosiloxanes mentioned in U.S. Pat. No. 5,211,877 or the LC pigments disclosed in DE 42 407 43, different colors arise at processing temperatures of 130xc2x0 C., the usual processing temperature in the OEM painting of vehicle bodies, than on processing at 80xc2x0 C., the usual processing temperature for repair finishes.
This varying behavior of the LC pigment is generally caused by swelling of the pigment during production of paints and the de-swelling process during drying of these paints, i.e. by the action of solvents on the pigment. The swelling process, and the dissolving-out of uncrosslinked constituents from the pigment which may occur at the same time, modifies the structure of the pigment. Warming of the dried paint film results in pronounced relaxation of the helix of the cholesteric liquid crystal, i.e. the pitch of the helix is shortened. This results in a shortening of the central reflection wavelength, which is directly proportional to the pitch of the helix. The color of the pigment thus shifts toward shorter wavelengths. A clearly visually perceptible blue shift occurs. Depending on the selected processing temperature, i.e. the baking temperature of a paint containing LC pigments of this type, various degrees of relaxation of the helix in the pigments occur. This results in the undesired variation in hue with processing temperature.
EP 0 887 399 solves this problem by selecting crosslinkable LC organosiloxanes as pigment material which, after crosslinking, give LC pigments having maximum glass transition temperatures (Tg) of 80xc2x0 C. and low uncrosslinked, extractable contents. Thus, complete and uniform relaxation of the helix is achieved at all temperatures above Tg, which means that the same color is always obtained at these temperatures. The property profile of these organosiloxanes is achieved by appropriate choice of flexible structural units, so-called spacers, in the LC monomers on which the pigments are based. Although these organosiloxanes modified with flexible structural units have quite good thermostability as LC pigments in the crosslinked state in the processing range 80xc2x0 C.-130xc2x0 C., they have, independently thereof, strong swelling especially processing window, of 80xc2x0 C.-130xc2x0 C., since incorporation of the spacers lowers the crosslinking density. LC pigments comprising mixtures in accordance with EP 0 887 399 exhibit strong interactions at room temperature with solvents usually used in paints, which results in strong swelling of the pigments and consequently a clearly visually perceptible red shift of the pigment color. In addition, the organosiloxanes modifications disclosed in EP 0 887 399 cause a significant increase in costs due to additional complex synthesis steps.
EP 0 887 398 discloses preparation of a system containing known LC pigments in which visually perceptible color differences at different processing temperatures are not evident, through selection of a suitable matrix. This system has the disadvantage that the user does not have a free choice of matrix for the LC pigments, but is subject to restrictions in selecting the paint components (vehicles and solvents).
EP 0 760 836, which corresponds to U.S. Pat. No. 5,807,497, discloses interference pigments comprising liquid-crystalline side-chain polymers made from low-molecular-weight, monofunctional or difunctional acrylates which are distinguished by higher color brightness, higher crosslinking density and better chemical resistance in conventional paints as well as simple production compared with the LC pigments disclosed hitherto. These LC pigments likewise have inadequate thermostability in the processing temperature range which is relevant for paint finishes of from about 80xc2x0 C. to about 130xc2x0 C. This inadequate thermostability is evident from shifts in the central reflection wavelengths of greater than 2 nm.
An object of the present invention is to provide a mixture of crosslinkable, liquid-crystalline substances having a chiral phase (LC mixture) which is suitable for the production of thermostable pigments, films and effect coatings.
For the purposes of the present invention, an article, for example a pigment, a film, or an effect coating, is thermostable if it does not exhibit any visually perceptible color differences in a paint or plastic matrix over a temperature range of from 80xc2x0 C. to 130xc2x0 C.
A color difference is generally accepted by paint users as being visually imperceptible if the color difference xcex94E* in accordance with DIN 5033 and DIN 6174 between two paint samples produced at different temperatures is xe2x89xa62 (L. Dulog, H. Schweiger, Farbe and Lack, 1997, 30-44). In order to achieve this aim in bright pigments (reflection  greater than 25%) in the green color region, in which the eye is particularly sensitive to color differences, experience has shown that it is necessary for the central reflection wavelengths to differ by xe2x89xa62 nm in the paint samples processed at 80xc2x0 C. and 130xc2x0 C., respectively.
This object is achieved by an LC mixture containing polymerizable groups, where at least 90% of the polymerizable groups are part of molecules containing at least two polymerizable groups (crosslinker molecules), wherein from 3.2 to 15 mmol of polymerizable groups are present per g of LC mixture.
The LC mixture according to the invention preferably comprises, as substances to which the polymerizable groups are bonded, low-molecular-weight, oligomeric or polymeric substances or mixtures of these substances having a chiral phase. These chiral phases can also be thermotropic twisted nematic, smectic or discotic phases. They can also be substances having a thermotropic or lyotropic phase.
The LC mixture according to the invention particularly preferably consists of mixtures of
compounds of the general formula Y1xe2x80x94A1xe2x80x94M1xe2x80x94A2xe2x80x94Y2, in which Y1 and Y2 are identical or different and are polymerizable groups, such as, for example, acrylate, methacrylate, epoxide, isocyanate, hydroxyl, vinyl ether or vinyl ester radicals, and
A1 and A2 are identical or different radicals of the general formula CnH2n, in which n is an integer from 0 to 20, and one or more methylene groups may be replaced by oxygen atoms, and
M1 has the general formula xe2x80x94R1xe2x80x94X1xe2x80x94R2xe2x80x94X2xe2x80x94R3xe2x80x94X3xe2x80x94R4xe2x80x94, where R1, R2, R3 and R4 are identical or different divalent radicals selected from the group consisting of xe2x80x94Oxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94CONHxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94Cxe2x89xa1xe2x80x94C, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Nxe2x95x90Nxe2x80x94 and xe2x80x94Nxe2x95x90N(O)xe2x80x94, and R2xe2x80x94X2xe2x80x94R3 or R2xe2x80x94X2 or R2xe2x80x94X2xe2x80x94R3xe2x80x94X3 can also be a Cxe2x80x94C bond, and X1, X2 and X3 are identical or different radicals selected from the group consisting of 1,4-phenylenes, 1,4-cyclohexylenes, arylenes, or heteroarylenes having 6 to 10 atoms in the heteroaryl ring which contain 1 to 3 heteroatoms from the group consisting of O, N and S and which are substituted by B1 and/or B2 and/or B3, cycloalkylenes having 3 to 10 carbon atoms which are substituted by B1 and/or B2 and/or B3, where B1, B2 and B3 can be identical or different substituents selected from the group consisting of xe2x80x94H, C1-C20-alkyl, C1-C20-alkoxy, C1-C20-alkylthio, C1-C20-alkylcarbonyl, C1-C20-alkoxycarbonyl, C1-C20-alkylthiocarbonyl, xe2x80x94OH, xe2x80x94F, xe2x80x94Cl, xe2x80x94Br, xe2x80x94I, xe2x80x94CN, xe2x80x94NO2, formyl, acetyl, and alkyl, alkoxy or alkylthio radicals having 1-20 carbon atoms which are interrupted by ether oxygen, thioether sulfur or ester groups,
with chiral compounds of the general formula V1xe2x80x94A1xe2x80x94W1xe2x80x94Zxe2x80x94W2xe2x80x94A2xe2x80x94V2,
in which V1 and V2 are identical or different and are an acrylate, methacrylate, epoxide, vinyl ether or vinyl ester radical, an isocyanate radical, C1-C20-alkyl, C1-C20-alkoxy, C1-C20-alkylthio, C1-C20-alkoxycarbonyl, C1-C20-alkylthiocarbonyl, xe2x80x94OH, xe2x80x94F, xe2x80x94Cl, xe2x80x94Br, xe2x80x94I, xe2x80x94CN, xe2x80x94NO2, formyl, acetyl, and alkyl, alkoxy or alkylthio radicals having 1-20 carbon atoms which are interrupted by ether oxygen, thioether sulfur or ester groups,
A1 and A2 are as defined above, and
W1 and W2 have the general formula xe2x80x94R1xe2x80x94X1xe2x80x94R2xe2x80x94X2xe2x80x94R3xe2x80x94, where R1, R2 and R3 are as defined above and R2 or R2xe2x80x94X2 may alternatively be a Cxe2x80x94C bond, and
X1 and X2 are as defined above, and
Z is a divalent chiral radical from the group consisting of dianhydrohexitols, hexoses, pentoses, binaphthyl derivatives, biphenyl derivatives, tartaric acid derivatives and optically active glycols,
where the concentration of the polymerizable groups of which at least 90% are on molecules containing at least two polymerizable groups in the mixture as a whole is from 3.2 to 15 mmol/g.
The LC mixture according to the invention preferably comprises from 70 to 99% by weight of compounds of the general formula Y1xe2x80x94A1xe2x80x94M1xe2x80x94A2xe2x80x94Y2 defined above and from 1 to 30% by weight of compounds of the general formula V1xe2x80x94A1xe2x80x94W1xe2x80x94Zxe2x80x94W2xe2x80x94A2xe2x80x94V2 defined above, where the concentration of the polymerizable groups of which at least 90% are on molecules containing at least two polymerizable groups in the mixture as a whole is from 3.2 to 15 mmol/g.
Depending on the color setting, particular preference is given to mixtures comprising 90-95% by weight of the compounds of the general formula Y1xe2x80x94A1xe2x80x94M1xe2x80x94A2xe2x80x94Y2 and 5-10% by weight of compounds of the general formula V1xe2x80x94A1xe2x80x94W1xe2x80x94Zxe2x80x94W2xe2x80x94A2xe2x80x94V2, where the concentration of the polymerizable groups of which at least 90% are on molecules containing at least two polymerizable groups in the mixture as a whole is from 3.2 to 15 mmol/g.
In a particularly simple embodiment, the LC mixture according to the invention is a low-molecular-weight nematic, crosslinkable, unsubstituted hydroquinone dibenzoate derivative having an average molecular weight of 450-800 g/mol, preferably 460-625 g/mol, and/or a hydroquinone monobenzoate derivative having an averaged molecular weight of 200-400 g/mol, preferably 220-300 g/mol, in combination with a chiral compound having an averaged molecular weight of from 500 to 1000, preferably from 500 to 700 g/mol, where the average molecular weight of the mixture as a whole does not exceed 625 g/mol.
The unsubstituted hydroquinone dibenzoate derivatives and hydroquinone monobenzoate derivatives preferably themselves have a large liquid-crystalline phase width of 80xc2x0 C., preferably with a clearing point (Tc) of 150xc2x0 C. Mixture components having non-liquid-crystalline properties should not reduce the liquid-crystalline phase width of the mixture as a whole to less than 30xc2x0 C. The mixtures themselves may also have monotropic liquid-crystalline properties.
It is a further object of the present invention to provide highly crosslinked, thermostable LC films having a chiral phase and LC pigments having a chiral phase which, when converted into paint systems and other polymeric matrixes, for example liquid plastics, have central reflection wavelength differences of xe2x89xa62 nm over the usual processing temperature range of from 80xc2x0 C. to 130xc2x0 C., and exhibit a reduced interaction with solvents compared with known LC pigments at temperatures below this processing temperature range, in particular at room temperature. The reduced interaction is preferably evident in the form of reduced swelling and thus reduced red shift of the inherent color. These and other objects are achieved by LC films and LC pigments consisting of crosslinked LC mixtures according to the invention.
In order to produce the LC films and pigments of increased thermostability according to the invention, known low-molecular-weight, difunctional monomers of high equivalent weight are unsuitable. Although low-molecular-weight monofunctional or difunctional monomers of this type, as disclosed, for example in EP 0 749 466 or EP 0 760 836, predominantly have two polymerizable units for use in chiral nematic mixtures, their molecular weight is large in relation to the number of polymerizable units. For example, at an average molecular weight of the individual components of 650 g/mol and with two polymerizable units per molecule, the mixture contains only a maximum of 3.08 mmol of polymerizable units per gram. Although polymerization of such mixtures gives virtually complete crosslinking, the number of network intersections per volume unit, i.e. the crosslinking density, is too low to suppress solvent interaction of the crosslinked polymer sufficiently to ensure thermostability and consequently color constancy.
Only an LC mixture according to the invention enables the production of thermostable LC pigments which have central reflection wavelength differences of xe2x89xa62 nm over a processing temperature range of 80xc2x0 C.-130xc2x0 C. in a matrix such as a conventional polymeric vehicle.
LC films and LC pigments of this type can be produced from an LC mixture according to the invention by processes known from the prior art, as described, inter alia, in the specifications already mentioned. One process for the production of an LC film according to the invention comprises applying an LC mixture according to the invention in the liquid-crystalline state to a smooth substrate, aligning the LC mixture, three-dimensionally crosslinking the LC mixture by polymerization, and detaching the film from the substrate. The LC mixture according to the invention is preferably applied to the smooth surface in a thickness of from 3 to 15 xcexcm, more preferably from 3 to 6 xcexcm. The alignment can be carried out, for example, by shearing, employing a knife coater or roller.
The polymerization of the aligned LC material can be carried out in a manner known per se, for example by means of free radicals using commercially available thermal initiators, by means of electron beams or using UV light in combination with commercially available photoinitiators, or alternatively by addition or condensation reactions.
The crosslinking of the mixtures according to the invention in the chiral structural state is carried out by means of a polymerization reaction, which proceeds, depending on the type of polymerizable, polycondensable or addition polymerizable groups, in the form of a free-radical, ionic or metal-catalyzed polymerization or as a polycondensation reaction or as a polyaddition reaction.
The initiation of free-radical polymerization can be carried out thermally by means of corresponding initiators, by UV radiation using commercially available photoinitiators, or by high-energy radiation such as electron beams. An advantage of thermal free-radical polymerization or polymerization via electron beam curing is that light stabilizers, such as UV absorbers (UVAs) or free-radical scavengers (HALS) can also be added to the polymerizable mixture in order to stabilize the resultant pigments or films against UV light, for example for outside applications, without impairment of the polymerization conversion occurring, as is the case in UV curing, owing to the shielding of the photoinitiator by a UVA. There is consequently no reduction in the crosslinking density.
If the curing of the LC films is carried out by means of peroxides or electron beams, the LC mixture according to the invention preferably contains commercially available light stabilizers, such as UV absorbers or free-radical scavengers in a total concentration of from 0.5 to 5% by weight.
Besides the light stabilizers, the mixtures according to the invention can contain further conventional additives for oxidation inhibition, polymerization inhibition or additives for improving rheological properties. In addition, absorbing fillers, such as pigments or carbon black, and fluorescent dyes or pigments may also be present.
The invention furthermore relates to a process for the production of LC pigments according to the invention. A process of this type comprises comminuting an LC film according to the invention to give platelet-shaped particles, preferably platelet-shaped particles having a diameter of from 5 to 100 xcexcm, more preferably from 10 to 50 xcexcm. Pigments having these geometrical dimensions are preferred pigments. They can be separated by a particle size-selective method.
The invention thus also relates to coating compositions, in particular, paint compositions, comprising LC pigments or LC films according to the invention. The LC pigments or films prepared according to the invention are suitable for incorporation into any desired coating compositions, and are particularly suitable for incorporation into paints for vehicle bodies.
The invention furthermore relates to articles, in particular vehicle bodies, coated with coating compositions according to the invention.